Generally, in a mobile communication system, a broadcast overhead message includes information enabling a mobile station to stably receive and decode packet data channel or broadcast channel transmitted from a base station in case of performing a broadcast/multicast (hereinafter abbreviated ‘BCMC’) service in the mobile communication system.
In a mobile communication system supporting BCMC, a high data rate is required since multimedia data including audio and video needs to be transmitted. To perform the broadcast/multicast service, a packet data channel of a physical layer needs to support a high data rate.
To stably transmit multimedia data via the packet data channel in a radio environment having fading, Hybrid Automatic Repeat Request (hereinafter ‘HARQ’) system is applied. In HARQ, technical features of Forward Error Correction (hereinafter abbreviated ‘FEC) function and Automatic Repeat Request (hereinafter abbreviated ‘ARQ’) are combined together.
The HARQ system is explained in detail as follows. First of all, encoding is carried out on data to be transmitted using a channel coder having an error correction function, e.g., turbo encoder, and at least one sub-packet corresponding to one encoded packet is transmitted.
Once a transmitting side transmits a first sub-packet, a receiving side decodes the received first sub-packet. If the decoding is successfully completed, signal notifying a successful reception (acknowledgement; hereinafter abbreviated ‘ACK) is transmitted to the transmitting side. Meanwhile, if the decoding of the received first sub-packet fails, signal notifying failure of reception (negative acknowledgement; hereinafter abbreviated ‘NACK’) is fed back to the transmitting side.
In case of receiving the ACK signal, the transmitting side transmits a first sub-packet corresponding to a next packet. Meanwhile, in case of receiving the NACK signal, the transmitting side transmits a second sub-packet corresponding to the previously transmitted packet. In this case, the receiving side stores the first sub-packet in a buffer to raise a decoding success rate in a manner of performing decoding by combining the first and second sub-packets together.
FIG. 1 is an exemplary diagram for a HARQ implementing method on interlace structure. Referring to FIG. 1, a channel for packet data transmission can be implemented with a structure that each interlace is regularly repeated with each fixed time interval. In an example shown in FIG. 1, since a packet data channel includes four interlaces, one packet is transmitted using one of the four interlaces. Once an interlace to be transmitted is determined, a corresponding packet is transmitted via the determined interlace. This is explained in detail as follows.
Referring to FIG. 1, it is assumed that packet #0 is transmitted using interlace #0. A transmitting side transmits a first sub-packet corresponding to packet #0 to a receiving side via interlace #0. The receiving side receives the first sub-packet and then performs decoding on the received first sub-packet. As a result of decoding the received first packet, if the decoding fails, the receiving side feeds back a NACK signal to the transmitting side. The transmitting side having received the NACK signal transmits a second sub-packet corresponding to the packet #0 to the receiving side using the interlace #0. Having received the second sub-packet, the receiving side performs decoding by combining the second sub-packet and the first sub-packet stored in a buffer together. If the decoding fails again, the receiving side feeds back a NACK signal to the transmitting side.
Having received the NACK signal, the transmitting side transmits a third sub-packet corresponding to the packet #0 to the receiving side using the interlace #0 again. Such a process is repeatedly performed until an ACK signal is received or a critical count is reached. Thus, each sub-packet corresponding one packet is transmitted using the same interlace.
Broadcast/multicast data is transmitted via a packet data channel having the above explained interlace structure and one interlace includes at least one or more multiplexes. Preferably, one interlace includes four, eight or sixteen multiplexes. So, an interlace-multiplex pair is used to indicate that a packet is transmitted through which multiplex within which interlace.
For each interlace-multiplex pair, a burst length is determined. The burst length is determined by multiplying a number of sub-packets per packet, which is determined by a data rate, by a number of packets per buts to be transmitted. The interlace-multiplex pair consecutively occupies a specific interval of the same interlace as the burst length.
So, a packet data channel carrying broadcast/multicast data includes sub-channels defined by interlace-multiplex pair. In a base station, one logical channel including at least one broadcast/multicast service (hereinafter abbreviated ‘BCMCS’) flow is mapped to at least one interlace-multiplex pair.
To transmit information associated with interlace-multiplex pair to a mobile subscriber station, an overhead message is transmitted from a base station. The overhead message includes information indicating that BCMC service flows are transmitted through which interlace from a specific base station and information about multiplex burst length. In the related art, all information about burst length of each multiplex configuring one interlace is transmitted to increase a size of the overhead message.